The invention relates to a fuel injection system for an internal combustion engine in which fuel is metered out on the basis of the instantaneous engine rpm and the air flow rate. The control information is fed to electromagnetic injection valves in the form of signals whose duration determines the quality of injected fuel. In a known system of this type which will now be described with the aid of FIGS. 1 and 2, the air flow rate is measured by a mechanism 2 which receives air after filtering by a filter 1 in a location 5 within the induction tube of the engine. A baffle plate 5 assumes a relative position which depends on the amount of aspirated air and the air flow meter 2 generates appropriate electrical signals for processing by a subsequent circuuit to be described. Under conditions when the throttle valve 4 is abruptly closed or nearly closed, the inertia of the air then flowing through the baffle plate region causes a certain amount of the air to flow into the region A ahead of the now closed throttle valve. Accordingly, the pressure in region A rises, which causes a force to be exerted on the baffle plate 5 in the direction of the arrow B and thus to change its position in a manner which would normally signal a smaller air flow rate. In other words, for a certain amount of time, the device produces a sensor signal which is different from that corresponding to the actually aspirated air quantity. Only after a certain delay does the air flow rate meter return to its normal state. The events just described cause the air number, i.e., the ratio of air to fuel, of the mixture supplied to the engine to be shifted toward higher values, i.e., the mixture becomes leaner and may cause misfires or near misfires. This is especially awkward in an engine which already operates at relatively high air numbers. Thus, the condition described causes the engine torque to fall and only after a certain time delay at which the output signal from the air flow rate meter 2 returns to normal does the torque increase again. A vehicle which uses an engine supplied in this manner tends to jerk and hesitate and produces an unpleasant driving sensation for the operator.
When misfires actually occur, the exhaust gases assume undesirable characteristics, and the concentration of toxic components, which may be detrimental to catalyzers, etc., increases.
The curves in FIG. 2 illustrate the change of the air flow rate aspirated by the engine due to the vacuum in the induction tube as a function of the motion of the throttle valve 4 and also show the change of the output signal from the air flow rate meter 2.
When the throttle valve 4 is rapidly closed at time t.sub.o, the air flow rate does not immediately change, as shown in the middle curve, because the air volume in the intermediate region from the throttle valve 4 up to the inlet valves of the engine is relatively large. If the output signal from the air sensor 2 were to be that shown by the thin line, i.e., corresponding to the air flow rate change in the middle curve, the fuel injection system would receive adequate information. However, due to the above-mentioned reasons, the output signal from the air flow meter 2 is in fact equal to that illustrated in the lower full curve which implies that in the shaded region the fuel air mixture is shifted towards higher air numbers and thus is leaner than desired.
The air flow meter 2 is so embodied that the baffle plate 5 adjusts its angle, depending on the pressure exerted on it, in such a manner that, when the air flow rate increases, the opening angle increases and the output potential delivered by the air flow sensor 2 increases relative to a predetermined norm. In similar manner, the output voltage decreases when the air flow rate decreases.